Lawyers are often used by governments attempting to justify their actions, but the growth of international law holds out the hope that lawyers of all political persuasions may be able to agree on fundamental principles. Such hopes, however, are dashed by this book, which spouts dangerous, muddled nonsense about the necessity for "a new jurisprudence" to justify what is described as pre-emptive government action. A respectable attempt to justify the Iraq war or even the actions of the state of Israel (which the author, a Harvard law professor, obsessively admires) would have dealt with the existing law. Instead Alan M Dershowitz repeatedly confuses that which is allowed and that which he would like to see allowed.

Dershowitz quotes at great length an article by himself asserting, in relation to the killing by the Israeli government of the Hamas leaders Sheikh Yassin and Abdel Aziz Rantisi, that "no reasonable argument can be made that the decision to target these combatants ... is unlawful under the laws of war or under international law". Despite the liberal use of footnotes this breathtaking assertion is not accompanied by a single legal citation. There is an opaque attempt in the rest of the book to justify the designation of an unlawful combatant as being protected neither by the laws of war as a prisoner of war nor by human rights law as a suspected criminal - the legal black hole of Guantánamo Bay. However, it now appears that this legal black hole is a basis not just for detention but for assassination.

Dershowitz attacks Jack Straw for condemning the killings of the Hamas leaders on the basis that the British government, by its involvement in the war on terror, is committed to "the killing of terrorist leaders who threaten British interests". Nothing could speak more eloquently of the moral and legal abyss into which our government has led us by its support for President Bush's foreign policy. I remember feeling chilled when the innocuous John Kerry committed himself to "killing terrorists" during his second televised debate with Bush, and Dershowitz cites this, too, as proof of the acceptance of the need for government assassinations.

Elsewhere in the book Britain's liking for "pre-emptive detention" is placed approvingly alongside that of Israel. But Dershowitz is ill-informed. He dismisses the House of Lords ruling that the indefinite detention of terror suspects is unlawful by saying the British government "refused to release them" - not quite an accurate description of the subsequent history of control orders and certainly not an adequate response by a lawyer to the ringing tones of the judgments. Similarly, the refusal of the Supreme Court in the case of Rasul v Bush to accept it had no jurisdiction over Guantánamo detainees hardly merits a mention. In its concluding chapter the book goes so far as to suggest that theories of "chromosomal abnormality" should be pursued as predictive of violent crime to justify long-term detention.

Most good international lawyers (such as Philippe Sands, the author of the excellent Lawless World) would agree that there is a legitimate debate to be had about the legal justification for pre-emptive wars, although most would want it to buttress, not to dismiss (as Dershowitz does), the authority of the UN. There is the conundrum that while "hawks" want preemptive wars to justify preventive strikes against states with WMDs, "doves" want humanitaritan intervention to stop genocide. If one is being charitable to John Reid, an informed debate is all he was suggesting when he recently called for the re-examination of the laws of war. However, he would do well to consider the real agenda of those to whom Dershowitz is a cheerleader. There are a lot of hawks in this book (with a whole chapter on possible justifications for attacking Iran) and little sign of the doves. This may be why the author's view of the new jurisprudence he considers necessary amounts to little more than a nonsensical cost-benefit analysis coloured by his own highly contestable views of what is already acceptable. The chilling nature of the various misreadings of international law in this book and its moral equivalence add weight to calls for Britain to withdraw its support from US foreign policy and the promulgation of the legal ideology that underpins it.

· Louise Christian is a lawyer who has acted for Guantánamo Bay detainees and the family of James Miller, who was killed by the Israeli Defence Forces in 2003. To order Preemption for £14.99 with free UK p&p call Guardian book service on 0870 836 0875.